The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.
Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
Photoresist materials are coated onto the surface of a wafer by dispensing a photoresist fluid typically on the center of the wafer as the wafer rotates at high speeds within a stationary bowl or coater cup. The coater cup catches excess fluids and particles ejected from the rotating wafer during application of the photoresist. The photoresist fluid dispensed onto the center of the wafer is spread outwardly toward the edges of the wafer by surface tension generated by the centrifugal force of the rotating wafer. This facilitates uniform application of the liquid photoresist on the entire surface of the wafer.
During the photolithography step of semiconductor production, light energy is applied through a reticle mask onto the photoresist material previously deposited on the wafer to define circuit patterns which will be etched in a subsequent processing step to define the circuits on the wafer. Because these circuit patterns on the photoresist represent a two-dimensional configuration of the circuit to be fabricated on the wafer, minimization of particle generation and uniform application of the photoresist material to the wafer are very important. By minimizing or eliminating particle generation during photoresist application, the resolution of the circuit patterns, as well as circuit pattern density, is increased.
A reticle is a transparent plate patterned with a circuit image to be formed in the photoresist coating on the wafer. A reticle contains the circuit pattern image for only a few of the die on a wafer, such as four die, for example, and thus, must be stepped and repeated across the entire surface of the wafer. In contrast, a photomask, or mask, includes the circuit pattern image for all of the die on a wafer and requires only one exposure to transfer the circuit pattern image for all of the dies to the wafer. Reticles are used for step-and-repeat steppers and step-and-scan systems found in wafer fabrication.
Reticles must remain meticulously clean for the creation of perfect images during its many exposures to pattern a circuit configuration on a substrate. The reticle may be easily damaged such as by dropping of the reticle, the formation of scratches on the reticle surface, electrostatic discharge (ESD), and particles. ESD can cause discharge of a small current through the chromium lines on the surface of the reticle, melting a circuit line and destroying the circuit pattern. The terms “mask” and “reticle” shall be used interchangeably herein.
Reticles are transferred among various stations in a semiconductor fabrication facility in reticle pods, such as SMIF (standard mechanical interface) pods. SMIF pods are generally characterized by a pod door which mates with a pod shell to provide a sealed environment in which the reticles may be stored and transferred. In order to transfer reticles between a SMIF pod and a process tool in a fab, the pod is typically loaded either manually or automatically on a load port on the process tool. Once the pod is positioned on the load port, mechanisms in the port door unlatch the pod door from the pod shell such that the reticle may be transferred from within the pod into the process tool. Another mode of reticle transfer includes the use of a wheeled cart or vehicle which includes a frame and multiple shelves on which are supported the reticles.
Within a cleanroom environment, reticles are typically hand-carried from a stocker to a step-and-scan system which is used to transfer the circuit pattern image of the reticle to the wafer substrate. Typically, the step-and-scan system can only hold two reticles at a time, whereas a succession of multiple reticles may be used in the step-and-scan operations throughout a single day. Thus, many of the reticles must be placed on a reticle vehicle or other support to await the step-and-scan procedure.
Throughout the course of using, transferring or storing a reticle or photo mask, static electricity has a tendency to accumulate and form an electric field on the mask. The electric field attracts electrically-charged particles in the air to the mask or induces a neutralizing discharge reaction on the surface of the mask, burning or melting the mask pattern. Consequently, the circuit pattern image transferred through the mask can be distorted, compromising pattern reliability and causing severe yield loss.
To minimize damage to the reticles by electrostatic discharge (ESD) as the reticles are hand-carried or transported from the reticle stocker to the step-and-scan system, multiple ESD eliminators are provided in the cleanroom, typically beneath the cleanroom ceiling. As is well known, such ESD eleminators (also known as static eliminators or electrostatic eliminators) typically include a pair of discharge electrodes across which a high-voltage A.C. current is applied to generate ions of each polarity. Air is blown downwardly into the cleanroom environment through vents in the ceiling, and this air carries the ions generated by the ESD eliminators to the surfaces of the reticles and prevents the buildup of electrostatic charges which may otherwise discharge and damage the reticles.
One of the problems inherent in the conventional use of multiple ESD eliminators mounted beneath the ceiling of the cleanroom is that many areas in the cleanroom lack sufficient downflow of air to facilitate sufficient transfer of the neutralizing ions from the ESD eliminators to the surfaces of the reticles. This increases the likelihood of ESD-induced damage to the reticles as they are carried or transported from the reticle stocker to the step-and-scan system and as they await their turn for the step-and-scan procedure. Accordingly, a novel ESD-resistant photomask and method of preventing ESD-induced damage to a photomask is needed.
An object of the present invention is to provide a novel method for eliminating ESD-induced mask damage to a photomask.
Another object of the present invention is to provide a novel method which facilitates integrity in the transfer of a circuit pattern image from a photomask to a photoresist layer on a wafer by eliminating or substantially reducing damage to the photomask caused by electrostatic discharges.
Still another object of the present invention is to provide a novel method for preventing ESD-induced damage to a photomask, which method includes implanting ions into the substrate of the mask to dissipate electrostatic charges on the photomask.
Yet another object of the present invention is to provide a novel ESD-resistant photomask which includes a substrate, a pattern-forming material provided on the substrate, a circuit pattern image provided in the pattern-forming material, and ions implanted in the substrate throughout ion implantation regions to dissipate electrostatic charges.